1. Field of the Invention
The present invention relates to a display device and a method of driving a display device, and more particularly, to a liquid crystal display (LCD) device and a method of driving an LCD device.
2. Discussion of the Related Art
Cathode ray tubes (CRTs) are used for display devices such as televisions and monitors. However, CRTs have some drawbacks such as heavy weight, large volume and high driving voltage. Accordingly, flat panel display (FPD) devices having advantages of portability and low power consumption have been the subject of much recent research and development for the coming information era. Among the various types of FPD devices, liquid crystal display (LCD) devices and electroluminescent display (ELD) devices are widely used as monitors for notebook computers and desktop computers because of their high resolution, ability to display colors and superiority in displaying moving images.
In general, an LCD device includes two substrates facing each other. A liquid crystal layer is provided between the two substrates, each of which has a electrode. When a voltage is applied between the two electrodes, an electric field is generated. The electric field modulates the light transmittance of the liquid crystal layer by reorienting the liquid crystal molecules, thereby displaying images in the LCD device. On the other hand, an ELD device uses an electroluminescence phenomenon such that light is emitted when an electric field above a critical intensity is applied to a fluorescent material. The ELD device is classified into an inorganic type and an organic type according to source exciting carriers. An organic electroluminescent display (OELD) device is widely used because of its superiority in displaying full color images and moving images. In addition, the OELD device has advantages of wide viewing angle, high brightness and low driving voltage.
FPD devices such as LCD devices and OELD devices include a circuit unit converting RGB data and several control signals of an external driving system into proper electric signals, and a display panel displaying images using the electric signals. In general, the circuit unit, which includes a gate driver and a data driver, is formed on a substrate different from the display panel.
Recently, an active matrix type display panel where a plurality of pixel regions are disposed in a matrix configuration and a switching element such as a thin film transistor (TFT) is formed in each pixel region is widely used. The TFT is fabricated through repetition of photolithographic processes.
A part of a driving circuit may be formed at a periphery of the pixel region during the fabrication process of the TFT in the pixel region. In such a case, because the driving circuit is partially formed in the display panel without increasing the number of photolithographic processes, the fabrication cost can be reduced. Specifically, a gate driver having a relatively low driving frequency may be formed in the display panel with a high reliability.
FIG. 1 is a schematic plan view illustrating an active matrix type flat panel display device having a gate driver according to the related art.
As illustrated in FIG. 1, an active matrix type display device 10 includes a display panel 20 and a circuit unit 30 driving the display panel 20. The display panel 20 have a pixel array 22 including gate lines (not shown), data lines crossing the gate lines (not shown) to define a plurality of pixel regions, and a pixel TFT (not shown) connected to the corresponding gate and data lines, and a gate driver 24 including a plurality of driving TFTs connected to the gate lines. Because the plurality of driving TFTs are simultaneously formed with the pixel TFTs, an additional photolithographic process is not required. The circuit unit 30 includes a source circuit 32 generating several driving signals and a data driver 34 connected to the source circuit 32. The data driver 34 may be a tape carriage package (TCP) type where a driver integrated circuit (IC) 34a is formed on a flexible printed circuit (FPC).
FIG. 2 is a schematic block diagram illustrating the gate driver 24 of FIG. 1 according to the related art.
As illustrated in FIG. 2, the gate driver 24 includes a plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R,” and a clock line L supplying a clock to the plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R.” A plurality of gate lines “g1,” “g2” and “g3” of the pixel array 22 (of FIG. 1) are connected to the output terminals of the plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R,” respectively, and the plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R” sequentially supply output signals to the plurality of gate lines “g1,” “g2” and “g3”. Because each output terminal of the shift register stages is connected to an input terminal of the next shift register stage, the gate signal of each shift register stage is used as a start signal of the next shift register stage.
FIG. 3 is a timing chart showing output signals of the gate driver 24 according to the related art.
As shown in FIG. 3, the plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R” (of FIG. 2) sequentially supply output signals' “Vg1,” “Vg2,” and “Vg3” respectively to the plurality of gate lines “g1,” “g2” and “g3” (of FIG. 2). Accordingly, a plurality of pixel TFTs connected to the plurality of gate lines “g1,” “g2” and “g3,” are sequentially turned on. The gate driver 24 according to the related art generates output signals having a simple shape (a square wave), and the shape of the output signals may not be modified.
FIG. 4 is a schematic circuit diagram illustrating a gate driver using two-phase clocks in a display panel for a flat panel display device according to the related art, and FIG. 5 is a schematic timing chart showing signals input to and output from the gate driver of FIG. 4.
As shown in FIG. 4, the gate driver includes a plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R” using two-phase clocks “CLK1” and “CLK2”. Each of the shift register stages “SRS1R,” “SRS2R” and “SRS3R” includes a shift register unit “SRU1,” “SRU2” or “SRU3,” and first and second transistors “T1” and “T2” connected to each other in series and to the corresponding shift register unit. Output signals “Vg1,” “Vg2” and “Vg3” are output from connection portions between the first and second transistors “T1” and “T2” to a plurality of gate lines “g1,” “g2” and “g3” in a pixel array, respectively. In the first shift register stage “SRS1R,” the first transistor “T1” is connected to a first clock line “L1” and the second transistor “T2” is connected to a source voltage terminal (or a ground terminal). In the second shift register stage “SRS2R,” the first transistor “T1” is connected to a second clock line “L2” and the second transistor “T2” is connected to a source voltage terminal (or a ground terminal). Similarly, the first transistors “T1” of the plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R” are alternately connected to the first and second clock lines “L1” and “L2”, and the second transistors “T2” of the plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R” are connected to a source voltage terminal (or a ground terminal).
Gate terminals of the first and second transistors “T1” and “T2” are connected to Q terminal and Qb terminal of the corresponding shift register unit. When the Q terminal has a high state and the Qb terminal has a low state, the first transistor “T1” is turned on and the second transistor “T2” is turned off. Accordingly, each of the shift register stages “SRS1R,” “SRS2R” and “SRS3R” outputs a corresponding clock signal of one of the first and second clock lines “L1” and “L2” connected to the first transistor “T1” to the corresponding gate line “g1,” “g2” or “g3.”
As shown in FIG. 5, two-phase first and second clocks “CLK1” and “CLK2” of the first and second clock lines “L1” and “L2” alternate with each other. Because the Q1 terminal of the first shift register unit “SRU1” has a high state in response to a start signal and the second clock “CLK2,” the first shift register stage “SRS1R” outputs the first clock “CLK1.” When a shift register unit uses two-phase first and second clocks “CLK1” and “CLK2,” a state of Q terminal may be changed from high to low by a clock signal such as the output signal of the previous stage or the next stage.
FIG. 6 is a schematic circuit diagram illustrating a gate driver using three-phase clocks in a display panel for a flat panel display device according to the related art, and FIG. 7 is a schematic timing chart showing signals input to and output from the gate driver of FIG. 6.
As illustrated in FIG. 6, the gate driver includes a plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R” using three-phase clocks CLK1 to CLK3. Each of the shift register stages “SRS1R,” “SRS2R” and “SRS3R” includes a shift register unit “SRU1,” “SRU2” or “SRU3,” and first and second transistors “T1” and “T2” connected to each other in series and to the corresponding shift register unit. Output signals “Vg1,” “Vg2” and “Vg3” are output from connection portions between the first and second transistors “T1” and “T2” to a plurality of gate lines “g1,” “g2” and “g3” in a pixel array, respectively. In the first shift register stage “SRS1R,” the first transistor “T1” is connected to a first clock line “L1” and the second transistor “T2” is connected to a source voltage terminal (or a ground terminal). The first transistor “T1” is connected to a second clock line “L2” and the second transistor “T2” is connected to a source voltage terminal (or a ground terminal) in the second shift register stage “SRS2R”; and the first transistor “T1” is connected to a third clock line “L3” and the second transistor “T2” is connected to a source voltage terminal (or a ground terminal) in the third shift register stage “SRS3R.” In this manner, the first transistors “T1” of the plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R” are alternately connected to the first, second and third clock lines “L1”, “L2” and “L3”, and the second transistors “T2” of the plurality of shift register stages “SRS1R,” “SRS2R” and “SRS3R” are connected to a source voltage terminal (or a ground terminal).
Gate terminals of the first and second transistors “T1” and “T2” are connected to Q terminal and Qb terminal of the corresponding shift register unit “SRU1,” “SRU2” or “SRU3,” respectively. When the Q terminal has a high state and the Qb terminal has a low state, the first transistor “T1” is turned on and the second transistor “T2” is turned off in the shift register stage. Accordingly, each of the shift register stages “SRS1R,” “SRS2R” and “SRS3R” outputs a corresponding clock signal of one of the first, second and third clock lines “L1”, “L2” and “L3” connected to the first transistor “T1” to the corresponding gate line “g1,” “g2” or “g3.”
As shown in FIG. 7, the three-phase first, second and third clocks “CLK1,” “CLK2” and “CLK3” of the first, second and third clock lines “L1”, “L2” and “L3” alternately have a high state. Because the Q1 terminal of the first shift register unit “SRU1” has a high state in response to a start signal and the third clock “CLK3,” the first shift register stage “SRS1R” outputs the first clock “CLK1.” When the first shift register unit uses three-phase first, second and third clocks “CLK1,” “CLK2” and “CLK3,” a state of the Q1 terminal may be changed from high to low by the second clock “CLK2.” Accordingly, the second clock “CLK2” is used as a disable signal of the first shift register unit “SRU1.” When a shift register uses three-phase or greater clocks, the timing control of the Q terminal and Qb terminal for the state change between high and low becomes easier.
As discussed above, the shift registers according to the related art use two-phase, three-phase or four-phase clocks of a square wave shape. Accordingly, the output signals of the shift registers according to the related art have a simple shape. In addition, the shape of the output signals may not be modified once the related art shift registers are formed. Accordingly, a gate driver having a shift register according to the related art may not perform various functions. In order to perform various functions with the related art gate driver, a large number of thin film transistors are required, which reduces the reliability of the gate driver.